In the integrated circuit industry, electron microscopes are central to microstructural analysis of integrated circuit components. The quality of a finished integrated circuit is highly dependent on the measurement and control of the circuit's critical dimensions. Thus, it is very important to ensure that critical dimension measurements received from metrology tools, such as electron microscopes, are precise and accurate. Typically, in critical dimension analysis of an integrated circuit component an electron microscope measures the apparent width of a structure when determining its dimensions. The apparent width of the structure is compared to critical dimension specifications in order to determine the compliance of the integrated circuit component.
Unfortunately, there are disadvantages to using the typical apparatus and method, as the apparent width of a structure as reported by the measurement tool is often different from the actual width of the structure. In addition, the discrepancy between the actual width and the apparent width of the structure could fluctuate from day to day, as well as from tool to tool. Thus, the integrity of the data derived from such measurements is often called into question, and is difficult to rely on.
In an effort to overcome this problem, it is possible to use a calibration piece having a structure with a known size. The calibration piece is loaded into the measurement tool and measured at regular intervals, such as once each day. The difference between the apparent width and the actual width of the structure on the calibration piece is used as a correction factor for other measurements. Unfortunately, even this procedure tends to not have the desired accuracy in all situations.
For example, most scanning electron microscopes can provide very good information about the dimensions of integrated circuit features within the plane of a wafer but very little information about the three-dimensional structure of these features. To over come this, dual beam tools having both a focused ion beam (FIB) and a scanning electron microscope (SEM) can be used. Such a tool uses the FIB to mill away a trench in the wafer proximate a feature of interest. The trench exposes a cross-section of the feature. Such a cross-section can be used to measure the physical dimensions of features in the direction perpendicular to the plane of the wafer. The SEM can be used to observe the cross-section and measure the size of the features in the horizontal and vertical directions. The electron beam from the SEM forms an angle that is approximately 45 degrees with respect to the plane of the wafer. Therefore, the image obtained from the SEM must be scaled by the cosine of the angle of incidence to obtain the actual size of the features. If the angle of incidence is not accurately known, the size of the features is in turn not known with accuracy.
FIGS. 1A-1B illustrate the problem. FIG. 1A shows a cross-section of a sample wafer 102. The sample wafer cross-section is viewed as shown in FIG. 1B. The FIB forms a trench 104 in the wafer 102. The trench 104 runs perpendicular to the plane of the drawings. The trench 104 exposes a sidewall 106 showing layers that make up features on the wafer 102. The features depicted in FIG. 1A appear on the sidewall 106. The view shown in FIG. 1A is taken assuming a particular angle of view θ. The angle of view θ is defined as the angle the SEM electron beam (indicated by the arrow) makes with respect to the plane of the wafer 102. A given feature has a thickness d′ when measured by the SEM. The measured feature thickness however is the height of the projection of the feature onto the viewing plane, which is perpendicular to the direction of the beam. The actual feature thickness d can be determined from the known viewing angle θ as follows:d=d′/cos θ
Note also the slant in the FIG. 1A, which begs the question as to whether it is real, or an artifact from the machine. A known standard may help answer the question. Unfortunately, there are no available standards and calibration techniques for calibrating such measurements.
Thus, there is a need in the art, for a method for calibrating vertical dimensions in dual beam FIB/SEM systems against a standard and a corresponding standard.